In 8 patients undergoing excision of epileptogenic lesions, 70 observations were made. In 42 instances NADH levels changed 3-15 percent in reponse to cortical stimulation. Studies of spread of NADH change indicate that this does not extend outward symmetrically from the region of stimulation but may vary from area to area in an asymmetrical fashion. These variable responses may reflect local tissue changes related to the epileptogenic process.